This disclosure relates generally to the field of imaging systems, and more particularly, to systems and methods for real time optical compensation of orbit induced distortion effects in long integration time imagers
Space-based Low Earth Orbit (LEO) imaging systems, as well as airborne imagers that are Earth-looking, have certain limitations to the length of integration time that they can gather the target signal. Chief among these limitations has been the changing of the projection of the sensor focal plane array (FPA) onto the Earth as the sensor moves along in orbit. Even with perfect stabilization of the field of view (FOV) center, the FPA can undergo a series of changes or distortions as it projects to the ground during the desired long integration. These series of distortions include: 1) rotation about the line of sight (LOS); 2) an overall and uniform expansion or compression of scale; 3) an anamorphic expansion and/or compression; and 4) a positive or negative stretch along one or both diagonals. All of these effects can cause many pixels of image smear on the FPA away from the FOV center and render the imagery useless.
Certain attempts have been made to optically address these distortion effects individually, as in only rotation effects, or only the uniform scale change, but no solutions are known that have addressed, even individually, the more difficult anamorphic or diagonal terms. Additionally, no previous attempts have been shown that are nearly indiscernible to the basic imaging properties of the optics; this is, that they effect distortion only, and not image quality.
Very-long integration times, on the order of seconds, needed to take “daytime quality” near-visible imagery at night are severely limited (˜100×) by orbit induced geometry or distortion changes which smear the image over large 2-D FPAs. The present disclosure addresses these issues by demonstrating optical correction of all distortion terms for relevant orbit parameters, at a level of 1%, without significant impact to system root-mean-square (RMS) wavefront error.
Even with perfect LOS stabilization (FOV center), platform motion during long integration times causes severe geometry changes for pixels away from the FOV center. These geometry changes act like “distortion” to cause image smear that increases as the distance from the array center. Because of this, integration time of a large visible staring array on an airborne or space platform in low light conditions is limited.